1. Field of the Invention
The field of the present invention is valve spring retainers for valve operating mechanisms for internal combustion engines, and particularly, lightweight valve spring retainers formed from aluminum alloys.
2. Description of the Prior Art
Such valve spring retainers have been conventionally made using a high strength aluminum alloy containing large amounts of Si, Fe, Mn, etc., added thereto, by utilizing a powder metallurgical technique.
However, the above aluminum alloy is accompanied by a problem: An initial crystal Si, an eutectic crystal Si, an intermetallic compound, etc., precipitated therein are very fine and hence, the resulting valve spring retainer may be subject to a large amount of slide wear and as a result, has a lacking durability under a higher surface pressure and under a rapid sliding movement.
There is also such a known valve spring retainer which includes a flange portion at one end of an annular base portion that has a diameter larger than the base portion, with an annular end face of the flange portion serving as an outer seat surface for carrying an outer valve spring and with an annular end face of the base portion serving as an inner seat surface for carrying an inner valve spring.
The valve spring retainer is produced utilizing a powder metallurgical technique and hence, the structure and the hard grain dispersion in a surface layer region having the outer seat surface are substantially identical with those in a surface layer region having the inner seat surface.
In the above valve operating mechanism, the outer valve spring has a relatively high preset load, while the inner valve spring has a relatively low preset load. Therefore, in the valve spring retainer, the slide surface pressure on the outer seat surface is larger than that on the inner seat surface. Under such a situation, and if properties of the outer and inner seat surfaces are the same, a difference in the amount of wear will be produced between the two seat surfaces, thereby bringing about a variation in load distribution between the outer and inner valve springs.
In addition, because a valve spring retainer is disposed in a limited space in the valve operating system, it is designed so that the thickness of the flange portion may be decreased to reduce the amount of projection in the direction of its valve stem. Therefore, there is a tendency to generate a concentration of stress at the junction between the flange portion and the base portion. Accordingly, it is desired to improve the fatigue strength of such junction.
Further, if hydrogen gas is included in the aluminum alloy, the fatigue strength thereof is damaged. Therefore, it is a conventional practice to subject a powder compact to a degassing treatment, but this treatment may causes not only a reduction in production efficiency for the valve spring retainer, but also a fear of damaging the strength thereof.